1. Field of the Invention
The present invention relates to circuits and methods for exciting a field coil of an electrical generator.
2. Description of the Related Art
Homes and commercial buildings commonly have an electrical backup generator system in which an internal combustion engine drives an electrical alternator that provides electricity when power is unavailable from an electric utility company. A controller responds to the interruption of the utility company power by automatically starting the engine. When the alternator reaches operating speed, the controller activates an automatic transfer switch to disconnect selected electrical circuits within the building from the utility power lines and connect those circuits to the output of the alternator. When the controller senses restoration of the utility company power, the automatic transfer switch is operated to reconnect the building electrical circuits to the utility company lines and thereafter stop the engine.
The alternator has a rotor with a field coil that produces a magnetic field which rotates with the rotor. The rotating magnetic field induces current in three stator windings to produce three phases of output current from the alternator. The field coil is energized by an exciter that supplies DC power which is effective to produce the magnetic field.
The DC power can be supplied to the field coil in several ways. One technique involves transferring the DC power from a stationary element to a rotating element on the rotor. This may be accomplished by feeding the DC power from the stationary element to brushes that contact slip rings on the rotor. Another technique is conventionally known as a “brushless exciter.” In one version, a DC current is applied to a stationary exciter winding that results in creation of an excitation magnetic field. One or more excitation windings on the rotor pass through this excitation magnetic field, thereby inducing alternating current in the excitation windings. The alternating current is rectified on the rotor to produce the required DC excitation current for the rotor field coil.
The output voltage produced by the alternator typically is regulated to a constant magnitude by selectively varying the DC supply current applied to the brushes or the stationary exciter winding. The alternator output voltage is sensed and any deviation from the desired magnitude is indicated by an error signal. A regulator circuit responds to the error signal by altering the DC supply current until the error signal indicates the desired output voltage is being produced.
Another technique for supplying DC power to the field coil employs a permanent magnet generator. A stationary permanent magnet assembly produces an excitation magnetic field. Excitation windings are rotated through (passed through) the excitation magnetic field thereby inducing an alternating current in the excitation windings. The alternating current is rectified on the rotor to produce the required DC excitation current for the rotor field coil. Because the excitation magnetic field is provided by a permanent magnet assembly, regulation of the alternator output voltage cannot be accomplished by controlling the excitation magnetic field. Instead, the application of power from the excitation windings on the rotor to the field coil has to be controlled to regulate the alternator output voltage.